The present invention relates generally to reflective articles. The invention has particular application to retroreflective sheeting fabricated using microreplication techniques.
"Retroreflective" as used herein refers to the attribute of reflecting a light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 5,066,098 (Kult). Due to the symmetrical geometry of beaded retroreflectors, microsphere-based sheeting exhibits a relatively orientationally uniform light return with respect to rotations about an axis normal to the surface of the sheeting. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
U.S. Pat. No. 4,708,920 (Orensteen et al.) discloses a modified beaded sheeting wherein a set of axial markings are formed in the sheeting by laser irradiation at a specific angle, each marking being located at the rear of a microlens. The sheeting thus bears a directional half-tone image composed of the axial markings and viewable, in only a selected cone of observation, in retroreflected light. Directional images such as this are widely used as anticounterfeiting measures for motor vehicle license plates.
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar front surface and a structured rear surface comprising a plurality of cube corner elements. Each cube corner element comprises three approximately mutually perpendicular optical faces that intersect at a cube apex or, where the cube apex is truncated, that otherwise converge at an uppermost portion. Examples of various cube corner designs include those of U.S. Pat. Nos. 1,591,572 (Stimson), 4,588,258 (Hoopman), 4,775,219 (Appledorn et al.), 5,138,488 (Szczech), 5,450,235 (Smith et al.), and 5,557,836 (Smith et al.). It is known to treat the structured surface with a specularly reflective coating to improve performance at high entrance angles. It is also known to apply a seal layer to the structured surface in a regular pattern of closed polygons which form isolated, sealed cells to keep contaminants away from individual cube corners. Heat and pressure used to form the cells destroys or deforms cube corner elements located along the polygon boundaries.
Cube corner sheeting typically has a much higher retroreflectance than beaded sheeting, where retroreflectance is expressed in units of candelas per lux per square meter. Cube corner sheeting therefore typically appears brighter than beaded sheeting in retroreflected light. However, certain graphics applications require not only high retroreflectance but high daytime "whiteness". The whiteness of an object is sometimes described in terms of the second of the tristimulus coordinates (X,Y,Z) for the object, and thus is referred to as "cap-Y". The cap-Y scale ranges from 0 for a perfectly black object to 100 for a perfectly white object. The whiteness of an object is also sometimes described in terms of its "Luminance Factor", ranging from 0 to 1. If the daytime whiteness of cube corner sheeting could be increased, without substantially reducing retroreflectance, such sheeting could find much broader application in graphics applications. Cube corner sheetings which have an aluminum or other metal vapor coat applied to the structured surface tend to have a somewhat grayish appearance, and could particularly benefit from an increase in whiteness.
It would also be desirable to incorporate directional images, such as are currently produced in beaded sheeting, in cube corner sheeting, even though conventional cube corner sheeting does not incorporate any lens- or microlens-type structure.
Cube corner sheeting is typically composed of a polymeric material formed using a precision negative mold having a mold structured surface which is the inverse or complement of the desired structured surface of the sheeting. There are two ways of obtaining the precision negative mold. The standard way begins with fabricating a "master mold" by a very expensive and time-consuming process involving precisely machining microscopic angled surfaces in a substrate such as a directly machinable substrate, individual pins, or one or more lamina. See, e.g., U.S. Pat. No. 4,478,769 (Pricone et al.). A second way, which is possible only if a sample is available having the desired structured surface in an undisturbed condition, is to use such a sample itself as a "master" from which replicas are made, thereby bypassing the expensive procedure of fabricating the master mold.
It would be desirable to mark cube corner sheeting in some way, preferably without substantially reducing retroreflective performance, in order to deter unscrupulous copyists from duplicating the structured surface of the sheeting without investing in the tooling and machining required to produce a master mold.